In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, and caliper, i.e., thickness, of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendaring rollers at the end of the process. Papermaking devices are well known in the art and are described, for example, in Handbook for Pulp & Paper Technologists 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and Pulp and Paper Manufacture Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. No. 5,539,634 to He, U.S. Pat. No. 5,022,966 to Hu, U.S. Pat. No. 4,982,334 to Balakrishnan, U.S. Pat. No. 4,786,817 to Boissevain et al., and U.S. Pat. No. 4,767,935 to Anderson et al.
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The papermaking machine is essentially a de-watering system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
U.S. Pat. No. 5,891,306 to Chase et al. describes a sensor that measures water weight on the wire of a papermaking machine. The sensor detects changes in resistance of the wet stock between the electrodes in an electrode array. The resistance of the wet stock between the electrodes is dependent on the amount of water above the electrodes, i.e., the water weight, and on the conductivity of the water. Since the conductivity of the water changes from time to time, the resistance measurement does not uniquely determine the amount of water unless some correction for the conductivity is provided. Consequently, the sensor also includes a separate reference cell which is designed to cancel out all affects that change the resistance between the electrodes other than the water weight. For instance, the resistance measurement is affected by changes in conductivity due to changes in the wet stock temperature or chemical composition.